Russian opposition activist Vladimir Kara-Murza Jr. has fallen gravely ill in Moscow for the second time in two years, and his wife has said doctors' preliminary diagnosis this week was the same as the first: "poisoning" by an unidentified substance.

Kara-Murza previously said he believed he was deliberately poisoned with a sophisticated toxin in May 2015 as retribution for his political activities, and his most recent illness has raised concerns that a similar attack led to his February 2 hospitalization and subsequent organ failure.

His case has also reverberated in Washington, where federal lawmakers have urged President Donald Trump's administration to prioritize Kara-Murza's plight as it formulates its Russia policy.

Here's what you need to know about the case.

Who Is Vladimir Kara-Murza?

Kara-Murza, 35, is a veteran politician who has been active in Russian liberal opposition parties and movements since President Vladimir Putin's rise 17 years ago. The son of a prominent journalist, also named Vladimir, he worked for several years as a television correspondent in Washington before joining political projects launched by former oil tycoon Mikhail Khodorkovsky, a prominent Putin foe who now lives in Europe after spending more than a decade in prison.

Kara-Murza was also a friend and ally of Boris Nemtsov, a prominent opposition leader killed in a February 2015 assassination-style shooting near the Kremlin. A dual Russian-British citizen -- he lived with his mother in Britain as a teenager -- Kara-Murza resides part of the year outside Washington, where his wife and three children live.

What Happened To Him?

In May 2015, Kara-Murza became suddenly and violently ill in Moscow. He had been conducting seminars and meeting fellow political activists in several Russian cities over the previous weeks. He was shuttled to various hospitals as doctors tried to determine what was wrong with him, and his major organs -- lungs, heart, kidneys, liver, intestines -- subsequently failed. He was placed on life support, and doctors determined he had suffered from acute poisoning "of an unknown origin," his wife, Yevgenia, told RFE/RL at the time.

Kara-Murza ultimately survived and spent months in treatment in Moscow and outside Washington. He suffered lasting nerve damage and had to walk with a cane following his hospitalization.

Was He Deliberately Poisoned?

This is unclear. If so, the substance and delivery mechanism remain a mystery. Kara-Murza's doctors in Moscow in 2015 concluded that he was "poisoned" by the widely prescribed antidepressant citalopram, which he had taken for several years. But an Israeli doctor consulted by Kara-Murza and his supporters viewed this diagnosis with skepticism, saying the near-fatal symptoms he suffered would be unusual for either an intentional or unintentional overdose of the drug.

Independent toxicologists who viewed Kara-Murza's medical documents also said citalopram was a highly unlikely culprit. An independent analysis of his blood, hair, and fingernail samples by a prominent French toxicologist led to no firm conclusions.

Vladimir Kara-Murza has had to walk with a cane since his last hospitalization in 2015. (file photo)

Kara-Murza believes he was targeted with a lethal toxin: "I have no doubt for a second that this was deliberate poisoning, that this was deliberate poisoning aimed to kill, and that it was motivated by my political activities," he told RFE/RL in a December 2015 interview. He added that he believes the toxin was likely a "very sophisticated" substance that typically only security services would have access to.

Several prominent government critics have fallen gravely ill or died in alleged deliberate poisonings during Putin's reign in what Kremlin opponents call a revival of Soviet-style techniques of dealing with dissent.

Russian officials dismiss such claims, including those concerning the 2006 poisoning death of former Russian security-services officer Aleksandr Litvinenko in London.

Why Would Anyone Want To Kill Him?

Kara-Murza is far from a household name in Russia, where the fractious liberal opposition has been not only steadily sidelined and demonized by the government and its media machine, but also mired in constant infighting. He has neither the fame nor the fiery public persona of opposition leader and anticorruption crusader Aleksei Navalny, but he is a dogged political organizer.

Perhaps more notably, he has the ears of prominent members of the U.S. Congress, where he has repeatedly lobbied for sanctions against senior Russian officials.

He was a prominent advocate in Washington for the Magnitsky Act sanctioning alleged Russian rights abusers, a 2012 law that has infuriated the Kremlin. He also urged Congress to sanction Kremlin-loyal television "propagandists."

Kara-Murza told RFE/RL in the December 2015 interview that he did not know of any specific threats against him prior to his illness but he believes his work for Khodorkovsky or Magnitsky Act lobbying were the most likely reasons for the alleged assassination attempt.

In February 2016, the volatile Kremlin-backed head of Russia's Chechnya region posted an Instagram video showing Kara-Murza and opposition politician Mikhail Kasyanov framed in what appeared to be a sniper's crosshairs

Kara-Murza's connections among Washington's political elite were evident after he was again hospitalized due to an apparent poisoning last week. Several leading U.S. lawmakers issued statements of support.

U.S. Senator John McCain (Republican, Arizona), Chairman of the Senate Armed Services Committee, on February 7 delivered a statement to the floor of the Senate honoring his "good friend," Kara-Murza. "Vladimir has once again paid the price for his gallantry and integrity, for placing the interests of the Russian people before his own self-interest," McCain said.

The same day, the top Republican and Democrat on the Senate Foreign Relations Subcommittee on Europe and Regional Security Cooperation urged Secretary of State Rex Tillerson to "pay close attention" to Kara-Murza's case as he reviews U.S. policy toward Moscow.

What Happened To Him Last Week?

Kara-Murza's wife, Yevgenia, told RFE/RL that he was at staying at her parents' house in Moscow on February 2 when he suddenly experienced symptoms similar to those of his 2015 poisoning. Over the previous weeks, he had traveled to several Russian cities to present a documentary about Nemtsov.

He was rushed to the hospital and again experienced organ failure, she said. He was placed in an induced coma and subsequently diagnosed with "acute poisoning by an undetermined substance," she said on February 6. Yevgenia Kara-Murza told RFE/RL that the "clinical picture is the same" as in 2015. As of February 7, he remained on life support in stable but critical condition, his lawyer, Vadim Prokhorov, said in a Facebook post.

Kara-Murza's abrupt illness has only heightened suspicions of his family and friends that he was targeted with poison. They say American doctors examined him exhaustively following his 2015 hospitalization and found nothing that would seriously imperil his health going forward. Furthermore, he stopped taking citalopram or any other medicine after the 2015 incident, his wife told RFE/RL on February 6.

Vladimir Kara-Murza and his wife, Yevgenia (file photo)

Samples of Kara-Murza's blood, hair, and fingernails have been sent to an Israeli lab for an independent analysis, his wife said. Prokhorov, his lawyer, told RFE/RL on February 7 that he was told it could take around 20 days for the results from the Israeli lab to be ready.

There's no guarantee, however, that those results will yield any firm conclusion about what triggered Kara-Murza's latest symptoms. Toxicology experts say determining a particular toxin -- particularly an exotic one -- through blood, hair, and fingernail analysis can be exceedingly difficult if it's not clear what one is looking for.

Are Russian Authorities Investigating The Case?

Russian investigators have looked into Kara-Murza's 2015 poisoning and are examining his latest illness as well, according to Prokhorov and media reports. But the lawyer has previously said authorities do not appear to be taking the case seriously. He previously based this assessment on the fact that it was being handled at a low level: the federal Investigative Committee's local branch in Moscow's Khamovniki district, which summoned Kara-Murza for questioning in January 2015.

Russia's state-run RIA Novosti news agency on February 7 cited an unidentified law-enforcement source as saying that the Investigative Committee is conducting a probe into his most recent illness "based on media reports."

Prokhorov said in a Facebook post the same day that he had received "confirmation from law-enforcement officers" of Kara-Murza's diagnosis upon his hospitalization: "toxic effect from an unidentified substance."